Families of the victims of the Lockerbie bombing have accused the British government of "betrayal" after it allowed Moussa Koussa, the former Libyan foreign minister, to leave the UK

Families of the victims of the Lockerbie bombing have accused the British government of "betrayal" after it allowed Moussa Koussa, the former Libyan foreign minister, to leave the UK to attend an international conference.

Koussa, who defected to Britain at the end of last month, was en route to Doha in Qatar on Tuesday, where an international conference on the future of Libya is to be held with representatives from the Benghazi-based opposition.

He is expected to return to the UK after the conference, but is free to travel as he pleases.

Brian Flynn, the brother of JP Flynn, who died in the 1988 attack and now organises the Victims of Pan Am 103 Incorporated campaign group in New York, said the UK authorities had "crossed a line" by allowing Koussa to attend the conference and thereby suggest he is a peace negotiator rather than, as they believe, a key instigator of the bombing.

"I think the British are being played by him … he has convinced them he can be valuable in this process, but he is not the suave diplomat in the suit sitting on the sidelines, he is one of the key guys who mastermined [the bombing of] Pan Am flight 103," Flynn said.

"He is a stated enemy of the British government. Our feeling is that the British government gave a nod to Lockerbie by questioning him two days before this conference, but that feels disingenuous. The Scottish and American prosecutors on Lockerbie are being betrayed by the politicians and the diplomats. Cameron has been good on Libya, but this sounds an awful lot like Tony Blair is back in charge."

Flynn's organisation, the largest victims' group in the US, seeks to discover the truth behind the bombing and win justice for those who died. He said the families believed the decision to allow Koussa to travel to the meeting in Qatar was part of a British strategy to encourage other defectors to flee to Britain from Gaddafi's regime, as there was no way either the rebels or the regime would trust him as an intermediary.

"He blatantly betrayed the Libyan regime and for more than 25 years he betrayed the Libyan people, so why is this the guy we are sending [to the talks]?" said Flynn.

Koussa is said to be travelling to Doha in order to establish whether he has a role to play in the rebel movement along with other senior defectors from the Gaddafi regime – perhaps by brokering a deal between Tripoli and Benghazi.

It is believed he has links with some leading rebel figures, including the opposition leader, Mahmoud Jibril.

It is understood Koussa spent a week being debriefed by MI6 at a safehouse before being allowed to go free. He was questioned by Dumfries and Galloway police about the 1988 bombing, in which 270 people died, though was he was not a suspect.

William Hague, the foreign secretary, had insisted that Koussa would not be given immunity from prosecution.

He was helped to defect by MI6 after leaving Tripoli for Tunisia on what was initially described as a private visit.

The hope in Whitehall is that Koussa's lenient treatment by the UK authorities will send a positive signal to other would-be Libyan defectors.

On Monday Koussa made his first public statement since leaving Libya 12 days ago. "I ask everybody to avoid taking Libya into civil war," he told the BBC. "This would lead to so much blood and Libya would be a new Somalia. More than that, we refuse to divide Libya. The unity of Libya is essential to any solution and settlement."